1. Technical Field
This invention relates generally to a method for controlling emissions from a diesel engine, and more particularly to a method for controlling smoke and/or NOx emissions during engine transients.
2. Background Art
In-cylinder water injection has been previously used in diesel engines. The most common approach has involved the use of emulsified fuels, stored in a tank or mixed in-line, and water injection through a nozzle separate from the fuel injector. When using emulsified fuels, the water content cannot be altered quickly enough for transient control. When injected separately, the water injection amount can be varied quickly, but this method does not reduce smoke (soot) emissions appreciably.
Water injection has been used for reducing steady-state emissions from diesel engines, but it has previously not been possible to vary the composition of a water-based diesel mixture with sufficient speed to enable transient emissions control. For example, U.S. Pat. No. 5,762,033, titled INJECTION DEVICE FOR COMBINED INJECTION OF FUEL AND SUPPLEMENTARY FLUID OR LIQUID was issued Jun. 9, 1998, to Helmut Rembold and Walter Teegen. That patent describes an injection device for combined injection of a fuel and supplementary liquid into the combustion chamber. The Rembold, et al. patent specifically describes a spark-ignited internal combustion engine having a distributor that rotates synchronously with the engine to provide the injection timing for the water. The Rembold, et al. system is specifically directed to continuous co-injection of water and fuel in a spark-ignited engine, although in the second paragraph of the specification, Rembold, et al. states that their injection device may also be used in self-igniting internal combustion engines although no teaching of how this may be accomplished is presented.
When water is injected continuously during engine operation, as taught by Rembold, et al., a significant amount of water must be provided. In vehicular applications, the large quantity of water that must be carried on-board adds significant weight and space requirements to the vehicle.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a system which only co-injects water with fuel when specifically needed to reduce NOx and smoke emissions, for example during engine transients. Moreover, by co-injecting water with fuel during engine transients, reduced air-to-fuel ratios can be used without excessive smoke emissions thereby allowing the engine torque rise rate to be improved. It is also desirable to have a water/fuel co-injection system in which the percentage of water in the mixture can be varied within a single combustion cycle.
In one aspect of the present invention, a method for reducing NOx and/or smoke emissions from a diesel engine during periods of transient changes in load demand include monitoring each combustion cycle of the engine during operation for a sensed value of at least one engine operating parameter representative of engine operation during the monitored cycle. The method further includes sensing a change in engine load demand, and determining an engine operating condition resulting from the change in load demand indicative of a high smoke and/or NOx condition. Water is then co-injected through an injector in fluid communication with a combustion chamber of the engine in response to the determined high smoke and/or NOx condition. The co-injection of water and fuel is terminated when it is determined that there is no longer an engine operating condition resulting from the sensing of a change in load demand that is indicative of a high smoke and/or NOx condition.
Other features of the method for reducing NOx in smoke emissions from a diesel engine during periods of transient changes in load demand include the sensed engine parameters being at least one of exhaust gas recirculation rate, injection timing, and manifold air pressure. Other features include the change in engine load demand being a change in torque demand, rpm demand, or intake manifold air pressure.
Still other features of the method include establishing a table of engine operating conditions at which high smoke and/or high NOx are produced and comparing the sensed values of at least one engine operating condition representative of engine operation during the monitored cycle with the established table.